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Measurement of Sivers asymmetry for di-jets at STAR in polarized p+p collisions at 200GeV

Measurement of Sivers asymmetry for di-jets at STAR in polarized p+p collisions at 200GeV. Outline Motivation Theory predictions Principle of measurement, L2 trigger Fast M-C model of Sivers spin asymmetry Measured SSA Conclusions. Jan Balewski, IUCF DIS 2007 April 16-20, 2007

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Measurement of Sivers asymmetry for di-jets at STAR in polarized p+p collisions at 200GeV

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  1. Measurement of Sivers asymmetry for di-jets at STAR in polarized p+p collisions at 200GeV • Outline • Motivation • Theory predictions • Principle of measurement, L2 trigger • Fast M-C model of Sivers spin asymmetry • Measured SSA • Conclusions Jan Balewski, IUCF DIS 2007 April 16-20, 2007 Munich, Germany

  2. Where does theproton’sspincome from? u u d pis made of 2uand1dquark S=½ =S Sq Explains magnetic moment of baryon octet p BUT partons have an x distribution and there are seaquarksandgluons Check viaelectron scatteringand findquarkscarryonly ~1/3of theproton’s spin! ½= ½DS+DG +Lq+Lg Di-jets, Sivers AN Jets,pions,ALL

  3. Motivation for pp  Di-Jet Measurement y proton spin x z Colliding beams parton kTx • HERMES transverse spin SIDIS asymmetries  u and d quark Sivers functions of opposite sign, different magnitude. • Sivers effect in pp  spin-dependent sideways boost to di-jets, suggested by Boer & Vogelsang (PRD 69, 094025 (2004)) • Both beams polarized, x +z  x z  can distinguish q vs. g Sivers effects. • Do we observe q Sivers consistent with HERMES? Tests universality. • First direct measurement of gluon Sivers effects.

  4. History of Predictions of Sivers AN for Di-jets @ STAR HERMES SIDIS  pp  dijet + X s = 200 GeV 8  pT(1,2)  12 GeV |(1,2)|  1.0 D. Boer & W. Vogelsang, PRD 69, 094025 (2004) – various models W. Vogelsang and F. Yuan, PRD 72, 054028 (2005). ISI+FSI AN, W=sin() Bacchetta, Bomhof, Mulders, Pijlman ISI ISI only SPIN 2006

  5. Lum. Monitor Local Polarim. Detector Barrel EM Calorimeter -1<η< 1 y STAR z Triggering Beam-Beam Counters 2<|η|< 5 h = - ln(tan(q/2) h=0 h= -1 -z beam +z beam h=2 Triggering Time Projection Chamber -2<η< 2 Endcap EM Calorimeter 1<η< 2 Tracking Solenoidal Magnetic Field 5kG

  6. L0 Jet Trigger +13,…,18 7 1 8 12 2 jet patch 3 size:1x1  x  11 9 4 10 Rate: 150 Hz, needs prescale by 20 or…smarter trigger 18 Hardwired 1x1 Jet patches  Rate: 3 Hz, sent to tape 4 8 1x1 Jet patch ET/GeV Luminosity 3*1031/cm2/s Allocated Jet Rate to tape only 15 Hz

  7. STAR Comparison with off-line  similar Zaxis=ET/GeV =360o 2005 data event Endcap EMC =[1,2] Barrel ||<1 =180o ~180 o back-to-back di-jet L2 algo result =0o  -1 0 +2 STAR Di-Jet Software Trigger (developed in 2006) • Rates @ STAR • 9.4 MHz : RHIC bunches collide • 300 kHz : min-bias collisions trigger BBCs • 150 Hz : L0 single jet exceeds 4 GeV ET • in a hardwired 1x1 ( x ) EMC patch • 10 Hz : L2 di-jet trigger with ET > 3.5 GeV • for a pair of back-to-back sliding 0.6x0.6 patches • ~80 muSec decision time

  8. STAR Di-Jets Acquired in 2006 run • 2006 p+p run, 1.1 pb1 • 2.6M di-jet triggered events • 2 localized clusters, with ETEMC > 3.5 GeV, | | > 60 Reco cos(bisector) measures sign of net kTxfor event spin Jet 1   LEFT Bisector RIGHT Jet 2 2 Is -distribution narrow enough for precise determination of centroid ? 1 X-axis  Signed azimuthal opening angle  Full, symmetric 1,2 coverage at STAR Sivers SSA

  9. Fast M-C Captures Essence of Sivers SSA jet 1 Back-to-back jets 1 spin bisector jet 2 avr kTx 2 Add Spin dependent kTX offsets y x Model Generator: • Back-to-back partonic collision yields a di-jet • intrinsic kT1,2 smearing • 2D Gauss(0,kTx, kTy) • sum kT from both IS • kT=kT1+kT2 • sum: • partonic pT + intrinsic kT

  10. Fast M-C Tuned to Real Di-jets y x  Fast M-C Data Fast M-C Data 

  11. Fast M-C: shift in equivalent to Sivers AN ‘Calibration’ of AN vs. amplitude Sivers <kTX> D. Boer & W. Vogelsang, PRD 69, 094025 (2004) - Assume non-zero Sivers <kTX>=100 MeV/c Displaced  peak for pol=+ and - SSA AN results naturally as the difference

  12. STAR Di-Jet Results Integrated Over Pseudorapidity Null Tests STAR data both jets rotated by 90 AN-z AN-z AN+z AN+z 2-spin 2-spin  rotation samples kTy, parity-violating sp•kT correl’n Error-weighted avg. of 16 independent AN(>) values for |cos(bisector)| slices, with effective  beam polarization for each = Pbeam |cos(bisector)| • Sivers asymmetries consistent with zero with stat. unc. = 0.002 • Fast M-C model  sensitivity to Sivers kTx offset  few MeV/c  0.004 (kTx) • Systematic uncertainties smaller than statistics • All null tests, including forbidden 2-spin asym.  cos(bisector), consistent with zero • Validity of spin-sorting confirmed by reproducing known non-zero AN for inclusive forward charged-particle production (STAR BBC’s)

  13. STAR Results vs. Di-Jet Pseudorapidity Sum Emphasizes (50%+ ) quark Sivers y Spin +z x +z Unpol qluon Pol quark STAR AN all consistent with zero  both quark and gluon Sivers effects ~order of magnitude smaller in pp  di-jets than SIDIS quark Sivers asym.! • V&Y calcs. include: • no hadronization • 5 < pTparton < 10 GeV/c • STAR  acceptance • HERMES-fitted q Sivers • no gluon Sivers fcns. They predict: • AN ~ ANHERMES where q dominates, if only ISI included (à la Drell-Yan) • AN sign reversal, factor ~2 reduction when gauge links include FSI as well Reverse V&Y signs here to obey Madison convention! Emphasizes (50%+ ) quark Sivers Emphasizes (80%+) gluon Sivers

  14. Theory Few Months Later AN , yields weighted w/ |sin()| ISI+FSI u-quark ISI+FSI u+d d-quark Bacchetta, Bomhof, Mulders, Pijlman ISI Bomhof, Mulders, Vogelsang, Yuan, hep-ph/0701277 31 Jan 2007 SPIN 2006 • Delicate cancellations of Sivers fcn for: • u and d quarks and • ISI and FSI • reconciles theory & experiment

  15. STAR Results : weighted AN  Di-jet yield AN  Di-jet yield W=|sin()| AN (weighted) Theory calculation for AN more robust, better characterization of initial state kT. Describes forward inclusive pi0's and analogous k_T-weighted SIDIS asymmetries moderately well =180 deg

  16. Summary & Outlook Full jet reco : charge tracks & EM • 2x larger statistics • Include HT & JP1 triggers • energy dependence • quark/gluon enhancement • u-,d-quark flavored jets • STAR acquired 2.6M di-jet events w/ transversely polarized proton beams at sqrt(s)=200 GeV • Experimental values of Sivers SSA for quarks & gluons is consistent w/ zero +/-0.002 • Early large predictions tuned to non-zero HERMES quark Sivers SSA has been reduced due to accounting for cancellation between ISI & FSI and u- and d-quarks Sivers fcns. • More detailed analysis is ongoing ….

  17. Backup Slides

  18. CNI Measured On-line 60%+ Polarization at RHIC in 2006 CNI pol  Year 2006  Year 2002

  19. 2006 pp200 Transverse Run @ RHIC May 9 April 6 WCM intensity spin Info problem STAR magnet incident ( ~10 days ) good STAR data good data days in 2006 STAR event count • Available STAR events: • both TPC & EMC active • few M 1-jet trig, ET>8.0 GeV • 3M di-jet trig, 2xET>3.7 GeV • few M di-jet w/o TPC • (crude run QA) Yellow Abort gap Blue Abort gap Yellow bXing (uses revTick) spin8 state STAR spin sorting (uses V124) • other value • B –Y – 51 • B –Y + 53 • B +Y – 83 • B +Y + 85 Example of di-jet events for one of 4 RHIC fill polarization patterns { \\ , \\+1, // , //+1 } Imperfections : few ppm

  20. Is the Di-jet Spin Sorting Correct? ( Find MinB BBC SSA) West BBC East BBC 2 6 6 2 Top view X Blue-Left Blue beam  +Z Y Yellow beam Yellow-Left Off-Line 12M MinB evens On-Line West-Blue 5.6  West-Yellow 0.3  YES ! Pol patt 1,2,3,4 East-Blue 0.1  East-Yellow 5.8 

  21. Theory - exp’t discrepancy raises questions! Drell-Yan S SIDIS l- p  * q q Initial state interaction p l+ ISI for Drell-Yan vs. FSI for SIDIS  opposite sign predicted for Sivers SSA • Are observed di-jet Sivers SSA much smaller than predictions because: • ISI & FSI both important in pp  jets and tend to cancel? • Need q Sivers or different q Sivers x, kT - shapes in HERMES fits? • Contributions from Collins fragmentation asymmetries in incompletely reconstructed jets conspire to cancel Sivers asymmetries for quarks, while gluon Sivers functions naturally small? • If ISI / FSI cancel at mid-rapidity, does their balance change at high  to yield sizable Sivers contribution to observed pp   0X SSA?

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